I think one of the easiest I have come across is "Advertisment Management" (note that is the exact name, some very similarly named plugins out there as well)
I don't think it is nearly as intensive as the ones mentioned in post above me, but it is great if you are looking for something simple. I have found it is perfect for people new to WordPress, and I usually recommend it to friends or clients new to Google ads.
Basically, it shows up in your WordPress settings and all it has is 8 fields, each field relates to an area on your blog (below post title, above header, etc...), you just throw your code into whichever field you want and the ad shows up.
It isn't the most flexible or option packed plugin - but for pure ease of use it works for me.